Mars is a world of giants. Some of the biggest volcanoes in the solar system rise from its desert landscape. Yet these mountains seldom exploded with fury, as many volcanoes do here on Earth. Instead, they built up over the millennia as molten rock bubbled gently to the surface.

Three of the five tallest volcanoes on Mars are in a region called Tharsis, while a fourth — the tallest of all — is just on its edge. All of these mountains are miles taller than Mount Everest. They appear to be extinct, although it’s possible that some of them could just be sleeping.

The biggest of the bunch is Olympus Mons — the tallest mountain in the entire solar system. It’s more than 15 miles high, and covers an area bigger than most American states. And although most of its slope is quite gentle, its edge is marked by cliffs that are up to four miles high.

Martian volcanoes grew so tall and wide because the lava that poured out of them was fairly thin. It spread out easily, so it covered large areas.

Many volcanoes on Earth are built by thicker lava, which quickly hardens. That allows pressure to build up below the surface, eventually causing an explosion — something that doesn’t happen with the gentle giants of Mars.

Look for Mars close to the lower left of the Moon before dawn tomorrow. It looks like a fairly bright yellow-orange star. The true star Regulus is below them. The Moon will stand side by side with Regulus on Tuesday.